According to Helmut Goersch, Wörterbuch der Optometrie [Optometry dictionary], 2nd edition, published by Bode GmbH & Co. KG, Pforzheim, 2001, page 26, corrective lens determination is the totality of all activities for determining the diopter combination (sphere, cylinder, prism) for correcting visual defects and for determining the near addition in the case of presbyopia. Corrective lens determination comprises a monocular part for determining ametropia (refraction determination) and presbyopia (short-vision lens determination) and a binocular part for determining heterophoria. Corrective lens determination therefore provides data which characterize the vision of the person being examined.
Corrective lens determination is an important part of eye examinations. During such corrective lens determination processes, visual defects are measured and can then be at least partially compensated for with visual aids such as glasses or contact lenses.
In the case of refraction determination, it is possible to distinguish between subjective refraction determination and objective refraction determination. In this case, methods for subjective refraction determination are based on (subjective) feedback from a person to be examined with respect to the person's visual perception. One example in this case is a measurement based on eye charts with an ever decreasing font or ever decreasing symbols, in which case the person to be examined provides feedback with respect to which characters can be discerned by the person. In contrast, methods and apparatuses for objective refraction determination do not require such feedback from the person to be examined with regard to the person's visual perception.
Photorefraction is such an objective refraction method. This method is based on a photo (that is to say an image recording) of light reflection in the pupil of a person. The refractive state can be calculated from the magnitude and position of this light reflection using a mathematical formula. According to Helmut Goersch, Wörterbuch der Optometrie [Optometry dictionary], 2nd edition, published by Bode GmbH & Co. KG, Pforzheim, 2001, page 239, a refractive state should be understood as meaning the state of the optical system of the eye with respect to its index of refraction in relation to its overall length.
W. Wesemann and H. I. Wesemann explain, in “Photorefraktion—Ein objektives Screening-Verfahren zur Refraktionsbestimmung [Photorefraction—An objective screening method for refraction determination]”, DOZ—Deutsche Optiker Zeitung, 11/92, pages 50 to 54, that a distinction is made between two physically/optically completely different methods in photorefraction, namely                1. the “isotropic” photorefraction developed by Howland and Howland (1974), and        2. the “eccentric” photorefraction presented by Kaakinen (1979).        
The “eccentric” photorefraction method is also referred to by other authors as “static photoretinoscopy” (Howland, 1980), “paraxial photorefraction” (Tyler and Noreia, 1985) and “photoretinoscopy” (Schaeffel et al., 1987; Howland, 1985).
W. Wesemann and H. I. Wesemann also state that the first eccentric photorefractometer, which was built by Kaakinen (1979), and also most subsequent devices (Hay et al., 1983; Molteno et al., 1983; Bobier and Braddick 1985; Day and Noreia, 1986; Noreia et al., 1986) comprised a single-lens reflex camera having a lens with a long focal length and a flash device. In contrast to a normal camera, however, the flash in eccentric photorefraction is fastened extremely close to the camera lens. The eccentricity of the light source is in this case the distance between the flash tube (light source) and the entrance pupil of the camera.
If an emmetropic eye is photographed using a flash mounted so close to the camera lens element, red eyes are obtained in the image. If a visually defective eye is photographed, a crescent-shaped light reflection additionally appears in the pupil, from the position and magnitude of which the visual defect can be derived. Further details are gathered from the publications cited above. The eccentric photorefraction method is also described, for example, under the URL roorda.vision.berkeley.edu/photoref.htm, version of Mar. 21, 2016, of Berkeley University.
DE 10 2007 031 923 A1 discloses an apparatus for eccentric photorefraction which uses a scanning light ray. The apparatus may be integrated in a surgical microscope, for example. Therefore, the apparatus described therein is suitable, in particular, for stationary use in doctor's surgeries or hospitals, for example.
A further apparatus for eye examination which can be fundamentally used—with slight modifications if appropriate—for objective refraction determination is described in US 2015/0002817 A1. This is a relatively complicated structure which is likewise predominantly intended for use in doctor's surgeries or clinics.
DE 197 19 694 A1 discloses a further apparatus for objective refraction determination using eccentric photorefraction. In this apparatus, light sources are arranged at different distances from a camera and are permanently installed with the camera in a common housing.
EP 1 308 128 A2 also discloses an apparatus for objective refraction determination using eccentric photorefraction. In this apparatus, all components required may be provided in a compact housing in order to therefore provide a mobile apparatus. A distance between the apparatus and a person to be examined can be determined by means of ultrasonic sensors, by means of optical triangulation, or by means of pattern projection, and this distance is then included in the refraction determination.
In the approaches described above, specially produced apparatuses are used for objective refraction determination and are accordingly expensive.
An attempt has increasingly been made in recent years to provide mobile, cost-effective possibilities for eccentric photorefraction determination. One approach is to use mobile computer apparatuses such as smartphones or tablets. Poster contribution no. D0055 at the ARVO (Association for Research in Vision and Ophthalmology) conference 2014, program no. 436, “Photoscreening for Refractive Error and Strabismus With a Smartphone App”, the abstract of which is published under www.arvo.org/webs/am2014/abstract/sessions/114.pdf, proposed carrying out an eccentric photorefraction measurement with a smartphone using an integrated flash of the smartphone and corresponding software (app). Commercial apps of this type for carrying out eccentric photorefraction are also available. Evidence is found, for example, under www.gocheckkids.com/ and www.gocheckkids.com/downloads/Poster_CEI_poster_APOS_Poster2014.pdf.
However, the use of an integrated flash light source of the smartphone may be disadvantageous in such an approach because, for example, the distance between the integrated flash light source and the integrated camera is unfavorable for the photorefraction measurement and/or the integrated flash light source emits in an unfavorable spectral range. In addition, the integrated flash light source is usually only in the form of an individual light source.
The distance between the integrated flash light source and the camera, together with other parameters, determines the measurement range in eccentric photorefraction.
The theoretical measurement range from −D to +D in diopters which can be achieved with an apparatus for eccentric photorefraction is described by the following formula:
  D  =            E              2        ⁢                                  ⁢        A        ⁢                                  ⁢        D        ⁢                                  ⁢        F        ⁢                                  ⁢        R              .  
In this case, E is the eccentricity of the light source, that is to say the distance between an effective light exit location of the light source and an entrance pupil of the camera. The effective light exit location of the light source is the location from which a light ray emerges in order to illuminate the eye. If the light source and the camera are in a plane perpendicular to the optical axis of the camera, the eccentricity corresponds to the distance between the center of the light source and the entrance pupil of the camera, in which case the center of the entrance pupil (usually on the optical axis of the camera) is used here. In the case of a conventional flash light source of a smartphone, the effective light exit location in this case matches the center of the flash light source. If the light source and the camera are not in such a plane, an offset between the light source and the camera in the direction perpendicular to the plane (vertical offset) is disregarded within the scope of this application. In this case, the vertical offset is typically considerably less than the eccentricity, for example less than 25% or less than 10% of the eccentricity. The eccentricity of the illumination should then be understood as meaning the distance between the optical axis of a camera lens of the mobile computer apparatus, which is intended to record a photo for determining the eccentric photorefraction, and a center of the effective light exit location of the light which is emitted by the light source and is intended to illuminate the person's eye while taking the photo. A is the distance between the eye and the camera. DF stands for the so-called “dark fraction”, that is to say the non-illuminated portion of the pupil, or in other words the portion of the pupil. R stands for the radius of the pupil which can likewise be gathered from the recorded image, for example (possibly scaled or normalized on the basis of the determined distance). It should be noted that deviations from this theoretical measurement range may arise in practice depending on the actual eye to be examined. In particular, the measurement range may be asymmetrical in practice.
As is clear from the above formula, the measurement range for a given distance is initially larger, the greater the eccentricity. However, in the case of large measurement ranges, the accuracy with which the eye refraction is determined for visual defects which are small in comparison with D falls, with the result that it may be desirable, for example, to use a smaller eccentricity for illumination. Minor visual defects can therefore be measured in a more accurate manner.
With respect to the use of a smartphone for objective refraction determination, DE 10 2015 100 147 A1 discloses an apparatus having a holder for accommodating a smartphone. The holder may be worn on the head. For objective refraction measurement, an image of the eye is recorded using a camera which is arranged on the same side as a display of the smartphone. This camera is usually referred to as a front camera. For illumination of the eye, different patterns can be displayed on the display of the smartphone. In addition, a light source can be provided in the interior of the holder, the light from which is centrally reflected into the beam path of the camera via a beam splitter.
DE 2 232 410 A discloses a distance measurement apparatus in which a prism generates a beam displacement.